High frequency jet ventilation and gas trapping

Abstract

We have compared three types of high frequency jet ventilation (HFJV) with conventional positive pressure ventilation in patients recovering from elective coronary artery bypass surgery. Twelve patients were allocated randomly to receive HFJV at ventilatory frequencies of 60, 100, 150 and 200 bpm from a standard jet ventilator at either the proximal or distal airway (HFJV.p and HFJV.d), or from a valveless high frequency jet ventilator acting as a pneumatic piston (VPP). Trapped gas volume (Vtr), cardiac index (CI) and right ventricular ejection fraction (RVEF) were measured. Vtr was related to the type of HFJV used (P < 0.05) and ventilatory frequency (P < 0.05). CI decreased with increasing rate of HFJV (P < 0.05) and there were significant differences between the three types of HFJV (P < 0.05). RVEF showed a linear relationship with ventilatory frequency (P < 0.05) decreasing most with the VPP. The decrease in RVEF was associated with an increase in right ventricular end-systolic volume (P < 0.05) suggesting that an increase in right ventricular afterload was the cause. The same three types of HFJV were compared using a lung model with variable values of compliance and resistance, to assess the impact of lung mechanics on gas trapping (Vtr, ml). Lung model compliance (C) was set at 50 or 25 ml cm H2O-1 and resistance (R) at 5 or 20 cm H2O litre-1 s, where values of 50 and 5, respectively, are normal. Vtr increased with ventilatory frequency for all types of jet ventilation (P < 0.05), varying with the type of jet ventilation used (P < 0.05).